1. Field of the Invention
The present invention relates to zoom lens systems. More particularly, the present invention relates to a zoom lens system suitable for use as an imaging lens included in a camera, such as a video camera, a surveillance camera, a digital still camera, a broadcast camera, and a silver-halide film camera.
2. Description of the Related Art
Recently, cameras, such as video cameras, digital still cameras, broadcast cameras, and silver-halide film cameras, including solid-state image pickup devices have become smaller with increased functionality.
Accordingly, demand has increased for small, high-zoom-ratio zoom lens systems having a small length for use in imaging optical systems of the cameras.
A four-unit zoom lens system including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power are arranged in order from the object side to the image side is known as a zoom lens system that complies with the above-mentioned demand.
An example of a four-unit zoom lens system is a so-called rear-focus zoom lens system in which the magnification is varied by moving the second lens unit and image-plane variation caused as the magnification is varied is compensated for by the fourth lens unit (refer to, for example, Japanese Patent No. 2002-182109, Japanese Patent Laid-Open No. 2002-287027, Japanese Patent No. 2000-89116 (corresponding to U.S. Pat. No. 6,118,593), and Japanese Patent Laid-Open No. 2000-227548.
In a rear-focus zoom lens system, an effective diameter of the first lens unit is generally small compared to that in a zoom lens system in which focusing is performed by moving the first lens unit. Therefore, the size of the overall lens system can be reduced. However, large variations in aberrations occur during focusing and it is difficult to obtain high optical performance over the entire object distance from an object at infinity to an object at a close distance.
Therefore, to obtain a small zoom lens system having a high zoom ratio, it is important to adequately set the zoom type, a refractive power of each lens unit, and the lens structure of each lens unit.
In particular, in the above-described four-unit rear-focus zoom lens system, it is important to adequately set the lens structure of the third lens unit and a back focus of the entire system.